It is well known to utilize a dual-limb ventilator or anesthesia machine to deliver a fluid, such as oxygen, air, or other breathing gas or gas mixture, to an airway of patient to augment, supplement, or substitute the patient's own ventilatory effort. For present purposes, the term “ventilator” is used to describe any system or device that delivers a flow of gas or pressurized gas to the airway of a user, alone or in combination with the delivery of other agents, such as anesthesia, supplemental gasses, aerosols, powdered medicaments, or any other material or fluid know to be deliverable to the airway of a patient. Of importance in such situations is the ability to accurately regulate or control the pressure, flow, and/or volume of gas exhausted from to the patient during the expiratory phase of the respiratory cycle.
In a conventional ventilator, the expiratory flow of gas from the patient enters the expiratory limb of the dual-limb circuit. The flow of gas exhausted from the expiratory limb is controlled in a number of ways using an exhaust valve. For example, it is known to use on/off exhaust valve or a proportional exhaust valve in the expiratory limb to control the flow of exhaust gas passing from the ventilator system. Controlling the flow of exhaust gas also controls the pressure in the ventilatory circuit.
In may instances, the exhaust valve is completely shut during inspiration, and completely open during expiration. Providing a relatively unobstructed (open) path during expiration maximizes the patient's comfort during expiration. In some situations, however, there is a need to maintain a certain pressure in the patient's lung at the end expiration. This final pressure may be necessary, for example, to keep the alveoli of the lung expanded so that they do not collapse. This final pressure at the end of expiration is typically referred to as the Positive End Expiratory Pressure (PEEP).
To maintain a certain PEEP, it is known to provide a pressure sensor in the expiratory limb, and regulate the actuation and/or position of the exhaust valve using a controller based on the output of the pressure sensor. In order to obtain a precise control of the expiratory flow/pressure, the controller is configured in a “closed loop” or “feedback” configuration using, for example, a PI or PID control technique as known in the art. By having control over the actuation of the exhaust valve, a ventilator has the ability to regulate the PEEP during expiration. In addition, the exhaust valve can be controlled during other portions of the breathing cycle, even during the inspiratory phase, as may be necessary or desirable depending on the ventilatory mode, pressure levels, or other conditions.
It is well established that it is important that the expiratory resistance is as low as possible, especially when the patient is breathing spontaneously. Therefore, exhaust valves are often made to as to have relatively large dimensions in order to minimize the pressure drop across the exhaust valve. However, larger dimensions for the exhaust valve make it harder to regulate PEEP, which requires controlling very small flow variations. The larger the valve, the harder it is to have a “fine tuned” control over the valve to maintain a precise PEEP level.
Another problem associated with regulation of PEEP in a conventional ventilator is that the control system is attempting to regulate the pressure for a relatively large volume, which has inherent instability. This volume includes the volume of the expiratory limb and the lung volume. The large volumes in combination with resistances and gas masses that have to be transported, leads to delays and instability. For example, the ability to control the pressure deteriorates as a due to the transit time that it takes for a pressure change to effect a large volume of fluid: the greater the volume of fluid, the longer the transmit time. In other words, the large the volume of fluid being controlled by the control system, the slower the system responds to pressure changes. In addition, the patient circuit (tubes) and the patient himself or herself have internal resistances and volumes that affect the ability of a pressure change induced by the valve to take effect in the whole system.
The tubes and the patient's respiratory system also include a certain amount of inherent flexibility, which is referred to as elastance, so that pressure changes cause the volume to expand or contract, thereby changing the volume on the control system. It can be appreciated that changes in the volume as the pressure is increased or decreased by the controller controlling the action of the exhaust valve make it harder for the control system to accurately control that volume to a certain PEEP. In addition, the fluid itself is compressible. This effectively results in low pass filtering of the pressure generation between the valve and the pressure transducer. Thus, the exhaust gas control system has difficultly accurately and quickly controlling the pressure in a stable manner.
A further problem associated with PEEP control that frequently occurs is that the building-up process towards the correct PEEP often includes pressure increases and decreases as the control system attempts to regulate the pressure to the correct PEEP. When the pressure decreases, gas is being removed from the system, if this occurs too rapidly, i.e., is not controlled within a tight tolerance, too much gas may be released and, in the worst case, may lead to alveoli collapse. This is particularly problematic, if the patient is a neonate or a small child with a small lung volume, which is quickly evacuated by a pressure decrease, i.e., by exhausting gas from the system. In other words, if the patient has a small lung volume, the pressure decreases must be tightly controlled, otherwise too much gas may be exhausted from the lungs leading to alveoli collapse.